You NEVER want to be around a girl when she does this, or ask her out around this special time. If you do, she will fucking bitch you out, cuss you out, possibly slap, hit, bite, scratch, you get what I mean. Then the worst thing is the next time you talk to her she acts like she doesn't remeber a fuckin' thing!

PMSing? Me? When!?! Never! Oh by the way sorry I slapped you, scratched your nutsac, ripped off your left nut, shoved my foot up your ass hole, shoved a hot iron down your throat, blew off your hand with a shotgun, twisted your nose with pliers and knocked you unconcious with my dad's PC. I was PMS - I mean, I was grumpy that day.

Usually before a woman begins her period, sometimes during and after though. She will usually be very emotional and will get angry or mood swingy at the littlest things.

"here is your coffee dear," "how many creamers did you put in it honey?" "I put two, just how you like it" "I LIKE THREE NOT TWO! YOU SHOULD KNOW THAT BY NOW! *starts to tear* YOU DON'T LOVE ME DO YOU!"
*1 week later* "sorry i kicked you out honey, i was PMSing"

Sometimes women turn into some type of mythical beast.
It's best to avoid them during this transformation..
Don't ever tell a girl she's PMSing though, it's never polite. =)
Just let the mythical beast be, and within a few days you'll get your girl back. ;)